1. Field of the Invention
The invention pertains to a process for producing a catalyst useful for the epoxidation of an olefin. More particularly, the invention pertains to an improved process for producing a catalyst useful for the epoxidation of ethylene to ethylene oxide. The process provides an improved catalyst that can reach its peak selectivity more quickly in the epoxidation process.
2. Description of the Related Art
There is interest in producing a catalyst useful for the epoxidation of olefins. Of particular interest are catalysts for the selective epoxidation of ethylene. These catalysts typically comprise a suitable solid support such as alpha alumina, which has on its surface an amount of silver and at least one promoter that helps to increase selectivity in the epoxidation process. The use of alkali metals and transition metals as promoters for silver catalysts is well known for the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase. Example catalysts are disclosed in U.S. Pat. Nos. 4,010,155; 4,012,425; 4,123,385; 4,066,575; 4,039,561 and 4,350,616.
Such highly selective catalysts contain, in addition to silver, selectivity-enhancing promoters such as rhenium, molybdenum, tungsten or nitrate- or nitrite-forming compounds, as discussed in U.S. Pat. Nos. 4,761,394 and 4,766,105. However, catalysts containing such selectivity-enhancing promoters often give comparatively low selectivities when first operated in the epoxidation process and may require long operating times to attain their peak selectivities.
As a matter of expediency, it is very desirable to shorten the time required for the high-selectivity catalyst to attain its peak selectivity. Elapsed time to peak selectivity can be shortened by conditioning the catalyst prior to use in the epoxidation process. According to WO 2004/002954 and WO 2004/002972, a pre-conditioned catalyst can give peak selectivity almost immediately upon feed introduction. These disclosures require the use of dry gas for catalyst conditioning, without the presence of steam. As specified, the dry gas contains oxygen and may also contain ethylene, carbon dioxide, and an organic halide modifier. Furthermore, the examples in these disclosures illustrate catalyst conditioning inside the reactor used for the epoxidation process, prior to feed introduction, rather than prior to placement in the epoxidation reactor.
The use of steam for conditioning catalysts has been disclosed by Mitsubishi in U.S. Pat. Nos. 4,690,913; 4,786,624 and EP 1201301 B1; wherein, superheated steam is used to treat the support after impregnation with silver and certain promoters. In these cases, steam treatment is used to remove volatile components from the impregnated support to produce the active catalyst. In these examples, steam is not separately used to condition the active catalyst.
The catalyst produced according to the current invention comprises a suitable solid support such as alpha alumina, which has on its surface a catalytically effective amount of silver and suitable promoters, including a selectivity-enhancing rhenium promoter and an alkali metal. The silver and promoters are first deposited on the surface of the support, and then the impregnated support is calcined to produce a catalyst precursor. Thereafter, the catalyst precursor is conditioned by contacting it with an atmosphere comprising a combination of oxygen and steam. The atmosphere during such conditioning is essentially free of olefins such as ethylene. This procedure affords a catalyst that attains peak selectivity in the epoxidation process much faster than a catalyst that has not been subjected to such conditioning in the oxygen and steam atmosphere.
It has now been unexpectedly found that a wet gas treatment of a high-selectivity catalyst, in particular, of a high-selectivity catalyst comprising silver, a rhenium promoter, and an alkali metal promoter, advantageously shortens the time required for the catalyst to reach peak selectivity in the epoxidation process. Furthermore, the activation conditioning step can be carried out in an ex-situ operation. The benefit of this discovery is that the catalyst can be used in the process without the complicated catalyst conditioning required by the prior art.